Fused salt electrolysis processes are used to produce metals. These processes can be distinctively divided into two categories. In one category a chloride is used as the feed material which is dissolved in a molten chloride based electrolyte and then decomposed electrolytically to produce metal on a suitable cathode and chlorine on a suitable anode. The typical example of this type of process is the electrolytic production of magnesium. In the second category, an oxide feed material is dissolved in a molten fluoride-based electrolyte and then electrolytically decomposed to produce metal on an appropriate cathode and oxygen on a carbon anode. The carbon anode reacts with the oxygen to form carbon dioxide and is consumed in the process. The typical example of this type of process is the electrolytic production of aluminum by the Hall-Heroult process.
No process is known heretofore in which a chloride feed material is dissolved in a fluoride-based electrolyte bath and then decomposed electrolytically to produce metal on a suitable cathode and chlorine on a graphite anode. Some advantages of such an approach would be: chlorides are likely to have higher solubilities in the fluoride melts in comparison with the limited solublities of oxides in these melts; the cell operating temperatures should be lower; and in some cases, energy consumption may be lower.
Aluminum has been traditionally produced by dissolving alumina in a cryolite based electrolyte and decomposing it electrolytically at about 1000.degree. C. ever since the invention of the Hall-Heroult process in 1886. The cell operates at a comparatively high temperature and produces fluoride compounds which pose serious health hazards.
A process using aluminum chloride as feed material and a chloride-fluoride mixed electrolyte for dissolving and electrolyzing the chloride feed, may eliminate both the problems mentioned above. In addition, there would be about 25% savings in the energy consumption for producing aluminum, as will be noted later on.
In the electrolytic production of aluminum according to the traditional Hall-Heroult process, alumina (Al.sub.2 O.sub.3) is used as feed material. It is prepared from bauxite. In the commonly used Bayer Process, bauxite is finely ground in a ball-mill and stored in large bins. The ground bauxite is next mixed with aqueous sodium hydroxide (NaOH) solution, of specific gravity 1.45, in a vessel fitted with stirrers. After intimate stirring, the mixture is pumped into steam-jacketed autoclaves and digested for 2-8 hours under a pressure of 50 to 70 lbs per square inch, at a temperature of 150 to 160.degree. C. The alumina of the bauxite reacts with NaOH, forming sodium aluminate which goes into solution. After the digestion is completed, the liquor from the autoclaves is forced into large iron settling tanks and held 4-5 hours to allow settling of solid impurities. This settled mass is called "red mud" and consists of undissolved alumina, ferric oxide, titania, silica etc., from the bauxite.
The sodium aluminate liquor from the tanks is diluted from specific gravity 1.45 to 1.23, filtered and run into large precipitation tanks to precipitate aluminum hydroxide. Sodium aluminate itself decomposes into aluminum hydroxide and sodium hydroxide, but the decomposition is accelerated by heating, stirring, and providing freshly prepared aluminum hydroxide seeds in the precipitation tanks. The aluminum hydroxide precipitation can take up to 60 hours. The aluminum hydroxide is separated by filtration and dried in cylindrical rotary kilns at 1000 to 1100.degree. C. to prepare substantially pure anhydrous aluminum oxide for use in the electrolytic cells.
The reduction cell or "pot" is a strong steel box, usually rectangular in shape, and is lined with refractory insulation, which surrounds an inner baked carbon lining. Thermal insulation is adjusted to provide sufficient heat loss to freeze an electrolyte coating on the inner walls, to protect them from the corrosive electrolyte attack. The bottom is not coated, and remains bare for electrical contact with the molten aluminum cathode. Steel collector bars are joined to the carbon container cathode, at the bottom, to conduct electric current from the cell. Current enters the cell through prebaked carbon anodes, or through a continuously self-baking Soderberg anode.
The carbon lined pot serves as the cathode. The lining is made by two methods. In one method, it is made by ramming a hot mixture of pulverized metallurgical coke with tar and soft pitch binders into the steel shell, using a suitable cast iron former to give the cavity the desired shape. The entire pot is baked in a furnace at about 600 to 800.degree. C. In the second method, preformed and prebaked carbon blocks are used to build up the lining, the blocks being cemented together with a mixture of tar, pitch, and ground coke. The cathode carries the current to the metallic aluminum. Overheating and local stresses cause it to crack and degenerate. Broken pieces of lining then float in the bath and cause partial short circuits between the anodes and the metal. Breaks in the lining permit the molten Al to attack the steel shell or collector box with the resultant dissolution of the iron by the Al. When this happens, the cell is removed and repaired. The carbon lining is porous and absorbs nearly its own weight of the fused electrolyte. When the cells are repaired, the old lining is broken out. The fused electrolyte is recovered by burning off the carbonaceous materials in multi-hearth furnaces. Cells may operate as long as 3 years, without repair or replacement.
The anodes are manufactured using calcined petroleum coke or hard pitch coke instead of foundry coke. Prebaked anodes are produced by molding petroleum coke and coal tar pitch binder into blocks (typically 70 cm.times.125 cm.times.50 cm) and baking to 1000-1200.degree. C. Steel stubs seated in the anode support the anode in the electrolyte and conduct electric current into the anodes. Electric resistivity of the anodes ranges from 5-6 .OMEGA.-m anode current density 0.65-1.3 A/cm.sup.2.
The Soderberg type of anodes are continuously baked. The anode briquettes are carried to the pots in portable hoppers, elevated and dumped into the top of the anode shell. The temperature at the top of the pot is high enough to melt the briquettes. The temperature in the lower part of the anode shell is high enough to distill the volatile matter in the briquettes and to bake the anode into a single solid block. The anode is lowered into the pot, at a rate which compensates for the carbon used during electrolysis. Its resistivity is about 30% higher than prebaked anodes. Current density is lower, ranging from 0.65-0.9 A/cm.sup.2.
It is believed that alumina dissolves in cryolite at low concentrations by the reaction, EQU Al.sub.2 O.sub.3 +4AlF.sub.6.sup.3- .fwdarw.3Al.sub.2 O F.sub.6.sup.2- +6F.sup.- (1)
and at higher concentrations by the reaction, EQU 2 Al.sub.2 O.sub.3 +2Al F.sub.6.sup.3- .fwdarw.3 Al.sub.2 O.sub.2 F.sub.4.sup.2- (2)
Ion transport measurements indicate that Na.sup.+ ions carry most of the current. This is consistent with the lower decomposition potential of sodium oxide (Na.sub.2 O) than that of aluminum oxide (Al.sub.2 O.sub.3) in these ionic melts. Al is deposited on the cathode by the reaction, EQU 12Na.sup.+ +4A1F.sub.6.sup.3- +12e.sup.- .fwdarw.12 (Na.sup.+ +F.sup.-)+4 Al+12 F.sup.- (3)
Oxyfluoride ions are discharged at the anode forming CO.sub.2 by the reactions, EQU 2Al.sub.2 O.sub.2 F.sub.4.sup.2- +C.fwdarw.CO.sub.2 +2Al.sub.2 O F.sub.4 +4 e.sup.- (4) EQU Al.sub.2 O F.sub.4 +Al.sub.2 O F.sub.6.sup.2- .fwdarw.Al.sub.2 O.sub.2 F.sub.4.sup.2- +2AlF.sub.3 (5)
Summation of equations (1) to (5) gives the overall reaction, EQU 2Al.sub.2 O.sub.3 +3 C.fwdarw.4 Al+3 CO.sub.2 (6)
Faraday efficiency is reported to be 85-95%. Loss of efficiency is reported to be caused by reduced species (Al, Na, or AlF) dissolving at the cathode and being transported toward the anode where these species are reoxidized by CO.sub.2, forming CO and metal oxide which can dissolve in electrolyte.
Aluminum is produced by carrying out electrolysis of alumina dissolved in cryolite (3NaF.AlF.sub.3) based electrolyte at about 1000.degree. C. using a carbon anode and a layer of molten aluminum as cathode. The electrolyte consists of cryolite, 4-8 wt % CaF.sub.2, 5-13 wt % AlF.sub.3, 0-7 wt % LiF, and 0-5 wt % MgF.sub.2. Joule heating from the flow of electric current is more than enough to maintain the melt temperature. Molten aluminum (sp. gr. 2.29 at 1000.degree. C.) is heavier than the electrolyte (sp. gr. .about.2.095 at 1000.degree. C.). Therefore, a layer of aluminum varying in thickness from a fraction of an inch to 4 to 5 inches serves as the cathode at the bottom of the cell. A layer of 6-12 inches of molten cryolite containing 2-5% alumina in solution serves as the electrolyte. The lower end of the carbon anode is kept 2-4 inches above the upper surface of the layer of molten aluminum. During the electrolysis, aluminum forms at the aluminum cathode, and oxygen forms at the carbon anode. Oxygen reacts with the carbon to form carbon dioxide.
Over the molten electrolyte is maintained a crust of frozen electrolyte mixed with alumina which is added as the feed. As the concentration of alumina in the electrolyte decreases by its consumption in the electrolysis, alumina is added to the electrolyte by breaking the frozen layer, on top of which has previously been distributed a layer of alumina. Alumina has to be fine enough to be maintained in suspension long enough to be dissolved by the agitation of the electrolyte. The time required for 200-mesh alumina to dissolve completely in fused electrolyte has been found experimentally to vary between 1.5 to 9 minutes, depending on the temperature, degree of saturation of electrolyte, and the character of alumina. The theoretical emf for the dissociation of alumina in the cryolite is 2.18 V at 1000.degree. C., but it becomes 2-3 times this value in the cell operation.
Cathode current densities vary between 300-600 A/ft.sup.2 (0.32 to 0.65 A/cm.sup.2) and anode current densities, based on the face area, vary between 5-7 A/in.sup.2 (0.77 to 1.1 A/cm ). Each cell takes a large current at a low voltage; a number of cells are arranged in a line in series. The line voltage may be 600-800 V. A line may contain 120 to 168 pots in series. Cell amperages are 34,000 to 130,000 amperes. Current efficiency varies between 90 and 95%. The power requirements for aluminum are 7.8 to 8.5 kWH dc or 8.3 to 9 kWH ac per pound. Carbon from 0.6 to 0.3 lb, alumina 2 lbs, and 0.03 to 0.05 lb of cryolite, are required for one pound of aluminum production. At 100% efficiency, 1000 amp, produces each 24 hr day, 17.746 lb Al and 21.689 lb CO.sub.2, equivalent to 800 cu. ft at 960.degree. C.
In 1973, Alcoa announced a new electrolytic process for producing aluminum, (the Alcoa process) using an alkali and alkali earth chloride-based electrolyte and aluminum trichloride (AlCl.sub.3 ) feed material. The process consisted of 1) production of very pure alumina by the Bayer process 2) chlorination of alumina for the production of aluminum trichloride and 3) the electrolysis of the aluminum trichloride dissolved in the electrolyte. Alumina with carbon was chlorinated in a reactor at 700-900.degree. C. The resultant aluminum chloride was purified and then stored in the crystalline state in a tank. The cell consisted of a steel enclosure, lined with a refractory material. The refractory material was believed to be silicon oxynitride. The metal produced was collected in a graphite compartment. The operating temperature was 700.+-.30.degree. C. A typical composition of the electrolyte is reported to be 5 wt % AlCl.sub.3, 53 wt % NaCl, 40 wt % LiCl, 0.5 wt % MgCl.sub.2, 0.5 wt % KCl and 1 wt % CaCl.sub.2.
Several bipolar electrodes were stacked in the cell on top of each other at an interpolar distance of approximately 1 cm. The anodically evolved chlorine was used to sweep the aluminum from the cathodes. The pumping effect of the chlorine bubbles also caused melt circulation and supply of new electrolyte to the electrode compartments. The aluminum settled in the bottom of the cell by falling counter-current to the chlorine gas. Alcoa reported a current density of 0.8-2.3 A/cm and a typical single-cell voltage of 2.7 V in comparison with the reversible decomposition potential of 1.8 V. The ohmic voltage drop in the electrolyte was about 0.5 V. The energy consumption was reported to be about 9 kWH/kg of Al including the chlorination step energy consumption.
Though the Alcoa chloride electrolysis process was theoretically promising, several difficult technical problems could not be solved satisfactorily. One difficult problem was the production and handling of the very pure and water-free aluminum trichloride. The process is no longer known to be in operation.